Combustion apparatus including an air-fuel premixing chamber

ABSTRACT

A combustor for generating a hot gas stream by the combustion of fuel in a pressurized airstream is disclosed including a generally toroidal-shaped combustion chamber of relatively short axial length, an annular orifice providing for the entry of a main airstream into the combustion chamber and a premixing chamber for generating and directing a vapor phase air-fuel premixture into the entering main airstream to form a combustible mixture for burning in the combustion chamber. In a preferred form, the mixture burns along a generally toroidal-helical gas flow path. The combustor is particularly suited for use in a gas turbine engine.

RELATED APPLICATIONS

This application is a continuation of applicant's application Ser. No.377,128, filed May 11, 1982, which in turn is a continuation ofapplicant's application Ser. No. 093,260, filed Nov. 13, 1979, which inturn is a continuation of applicant's application Ser. No. 877,897,filed Feb. 15, 1978, which is in turn a divisional of applicant'sapplication Ser. No. 707,326, filed July 21, 1976, for "CombustionApparatus Including an Air-Fuel Premixing Chamber", now U.S. Pat. No.4,084,371, which in turn is a continuation of application Ser. No.491,611, filed July 24, 1974, for "Combustor", now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an improved combustor for generating a hot gasstream by the combustion of fuel in a pressurized airstream. Morespecifically, this invention relates to an improved combustor capable ofgenerating a hot gas stream with minimum heat, pressure and velocitylosses and with minimum formation of noxious or undesirable exhaust gasconstituents.

The present combustor has a wide range of utility, e.g., in room heatingand powerhouse steam generating apparatus and is particularly useful ingas turbine engines of both the stationary industrial type and thepropulsive type. Although my invention will be described as it isapplicable to gas turbine engines for use in automotive and flightpropulsion, it is to be recognized that it is equally applicable to anysystem where fuel is burned in a pressurized airstream to generate a hotgas stream.

In gas turbine engines there are many desiderata for the combustion offuel for generating a hot gas stream. For example, for efficiency andeconomy of operation, a hot gas stream should be generated with minimumheat, pressure and velocity losses. Any pressure drop across thecombustor lowers the available pressure drop through the turbine and,consequently, the energy available for conversion to useful work in theturbine. For a given fuel flow, this decreases the turbine horsepowerand increases the specific fuel consumption Second, it is desirable thatthe hot gas stream be generated with minimum formation of undesirableexhaust gas constituents, e.g., unburned hydrocarbons, carbon monoxideand oxides of nitrogen. Third, it is desirable that a stable flame ofcontrolled temperature achieving complete combustion of the fuel bemaintained within the combustor. In addition, all of these desirableoperating characteristics should be obtained over a wide range ofambient pressures and temperatures and at rapid changes in fuel and airmass flow rates through the combustor. Moreover, it is highly desirablethat the combustor be compact, light and simple to manufacture andmaintain.

Efforts to achieve these desiderata in prior art combustors have notbeen completely successful. Indeed, these prior art combustors havesuffered from one or more serious disadvantages. By way of example,reverse flow-type combustors widely used in aviation and industrial gasturbine engines are wrapped around the outer diameter of the turbine tokeep the engine short and are therefore relatively large in diameter. Insuch combustors the fuel must be injected with a high velocity in orderto achieve fast and complete mixing and combustion. This turbulence isgenerated by a significant pressure drop in the combustor and additionalpressure drops result from the large housing and liner surfaces and theturns required to direct air into the inside of the liner. Moreover, thehot and cold air layers are far apart and must be brought together formixing. Thus, the combustor must be relatively long and the fuelinjectors must be placed at the far downstream end thereof. Moreover,the large inner surface absorbs a large amount of heat that must becarried away. And finally, in an effort to reduce weight and cost, theselarge surfaced combustors are welded from sheet metal, with the resultthat they are particularly susceptible to failure from vibration,thermal stress and stress concentrations Each of the foregoing is asignificant disadvantage.

Accordingly, it is among the principal objects of my invention toprovide an improved combustor of compact and simple construction whereinfast and complete mixing of fuel and air and efficient burning isachieved to generate a hot gas stream with minimum heat, pressure andvelocity losses.

It is a further object of my invention to provide an improved combustorof relatively short axial length which, nevertheless, provides forrelatively long gas flow path lengths and maximum flame stability andburning efficiency with a minimum of heat losses.

It is a still further object of my invention to provide an improvedcombustor wherein the fuel and pressurized air from the compressor arequickly and completely mixed to provide a substantially homogeneousvapor phase air-fuel premixture which is then thoroughly mixed into amain airstream to produce a combustible air-fuel mixture having adesired air-to-fuel ratio for complete combustion with minimum formationof undesirable exhaust gas constituents.

The present invention is predicated in part upon the concept ofproviding a combustion chamber in which the flame does not extend in anaxial direction, but rather is curved back upon itself in generally theshape of a segment of a toroid. A second important concept of thepresent invention is to provide a premixing chamber in which an intimatemixture of fuel and air is formed immediately adjacent to the combustionchamber in combination with means for injecting the air-fuel premixtureinto a main airstream entering the combustion chamber adjacent to theouter periphery thereof with the injected air-fuel premixture having asubstantial tangential component such that almost instantaneous, highlylocalized intermixing of the air-fuel premixture into the main airstreamis obtained to generate a combustible air-fuel mixture for burning inthe combustion chamber.

In the preferred embodiment of my invention, these and other objects areaccomplished by providing a combustor comprising a housing whichencloses an arcuate combustor liner effective to constrain the gas flowto form a generally toroidal combustion zone or chamber, an annularair-fuel premixing chamber upstream of, and opening downstream into, thecombustion chamber, and a plenum surrounding the premixing chamber forreceiving pressurized air from the compressor. The plenum is separatedfrom the air-fuel premixing chamber by a baffle and communicatestherewith through a plurality of spaced openings in the baffle angledgenerally tangentially to the inner surface thereof to provide for theentry of air to generate a vortically flowing, substantially homogeneousvapor phase air-fuel premixture in the premixing chamber. The plenumalso communicates with the combustion chamber through an annular orificeoutwardly circumferential to the baffle at its downstream end to providea main airstream entering the combustion chamber. In a preferred form,the main airstream has a vector component of flow effective to generatea helical gas flow path through the toroidal-shaped combustion zone. Thevortical discharge from the air-fuel premixing chamber enters the mainairstream and becomes thoroughly mixed therein, thereby generating acombustible air-fuel mixture having a predetermined air-to-fuel ratio atthe entrance to the combustion chamber for burning therein. Thetoroidal-helical flow of the combustible air-fuel mixture provides arelatively long gas flow path length in a compact housing. Therelationship of the flame which surrounds a vortical recirculation zoneof burned gases is effective to cause ignition and efficient and stableburning of the mixture

Other objects and advantages of my invention will be apparent from thefollowing description of a preferred embodiment embodying the presentinvention and several modifications, reference being had to theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified longitudinal section of a gas turbine engineincluding the combustor of the present invention.

FIG. 2 is an enlarged cross-section of the combustor shown in FIG. 1.

FIG. 3 is a view of the combustor similar to FIG. 2 illustrating thecombustion process occurring therein.

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 2.

FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 2.

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 3.

FIG. 7 is a plan view taken along line 7--7 of FIG. 5.

FIG. 8 is a cross-sectional view similar to FIG. 2 showing anotherembodiment of the combustor.

FIG. 9 is a cross-sectional view taken along line 9--9 of FIG. 8 showingthe openings in a full open position.

FIG. 10 is a cross-sectional view similar to FIG. 9 showing the openingsin a partially closed position.

FIG. 11 is a partial cross-sectional view showing a feature of thecombustor shown in FIG. 8.

FIG. 12 is a partial cross-sectional view of another embodiment of theinvention.

FIG. 13 is a cross-sectional view of another embodiment of theinvention.

FIG. 14 is a view taken along line 14--14 of FIG. 13.

FIG. 15 is a partial cross-sectional view of another embodiment of theinvention.

FIG. 16 is a cross-sectional view of another embodiment of theinvention.

FIG. 17 is a view taken along line 17--17 of FIG. 15.

FIG. 18 is a cross-sectional view of another embodiment of theinvention.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 shows the basic elements of atypical aviation gas turbine engine 10 incorporating a combustor of thepresent invention. Cold, fresh intake air enters an air inlet 11 of theengine 10 and travels to a compressor 12 located at the front end of theengine casing where power is employed to raise the pressure of theintake air. The pressurized intake air emerging from the compressor 12passes through a diffuser 13 into the combustor 14 where fuel is burnedto heat it to a high temperature. As shown in FIG. 1, the combustor 14is annularly disposed about the longitudinal axis 16 of the engine 10.The highly heated compressed gas then enters the turbine section whereit expands to a lower pressure and the energy released during thisexpansion is converted to useful work.

In the specific aviation turbine of FIG. 1, the hot compressed gasstream passes to a first turbine 18 which drives the compressor 12 andthen is ducted to a propulsive nozzle 20 In other designs, thecompressed gas stream may pass to a second turbine in series with thefirst where its remaining energy is utilized to drive the engine. Incombustors used for room heating or powerhouse steam generating, the hotcompressed gases are discharged from the combustor directly into the hotair or steam generator and the compressor is driven by an external powerunit, e.g., by an electric motor. It will be noted that the gas turbineengine illustrated in FIG. 1 is of the radial compressor type althoughit is to be understood that the combustor described is equallyapplicable to axial compressor systems.

Referring now to FIG. 2, the combustor 14 comprises a housing 22, acombustion liner 24 mounted within the housing 22 in spaced relationthereto defining a generally toroidal-shaped combustion zone or chamber28, and an interior housing liner 30 including annular baffle portion 26and an annular air-fuel premixing chamber 34 disposed radially inwardlythereof having a closed upstream end 36 and opening downstream into thecombustion chamber 28.

Surrounding the premixing chamber 34 and defined cooperatively by thehousing 22 and the baffle 26 is a plenum 38 for receiving thepressurized air from the compressor 12. The plenum 38 communicates withthe air-fuel premixing chamber 34 through a plurality of openings 40 inthe baffle 26. The plenum 38 further communicates with the combustionchamber 28 through an annular orifice 42 defined by the outercircumferential surface of the downstream end of the baffle 26 and theinner surface of the upstream end of the combustion liner 24.Accordingly, pressurized air traveling from the compressor 12 into theplenum 38 is split into a main airstream entering the combustion chamber28 through the annular orifice 42 and a secondary airstream entering thepremixing chamber 34 through the baffle openings 40.

The combustion liner 24 is axially located and centered in the housing22 by four radially extending pins 44 spaced evenly about thecircumference of the housing 22 (FIG. 6) and by a radially extendingignition plug 46. The pins 44 and plug 46 lie in a plane close to, andparallel to, a plane through the center of the combustor torus (axisy--y of FIG. 3). This method of attachment assures exact location of thecombustor liner 24 in the housing 22, while permitting free heatexpansion in all directions. The interior housing liner 30, includingthe baffle portion 26 and the annular premixing chamber 34, is radiallycentered on shoulders 48 in ribs 50 in the housing 22 and axiallylocated and secured in the ribs 50 and housing 22 by axially extendingbolts 52 (FIGS. 2 and 4). The housing ribs 50 coincide with the guidevanes of the radial compressor diffuser 13 and, accordingly, the bolts52 also secure the diffuser and compressor axially and radially withinthe engine casing. This method of attachment also assures exact locationand free heat expansion of the interior housing liner 30.

The housing ribs 50 also contain circumferentially spaced fuel passages56 which coincide with fuel entrance bores 58 opening into the air-fuelpremixing chamber 34 at the closed end 36 thereof and upstream of thebaffle openings 40. A suitable O-ring 59 seals the connection betweenthe passages 56 and the bores 58. A fuel control and metering device(not shown) external of the combustor supplies fuel to the premixingchamber at a pressure and velocity only high enough to maintain adesired fuel flow rate. A variety of suitable fuel metering devices areknown in the art, and since their exact construction constitutes no partof the present invention, it is considered unnecessary to describe thefuel metering device in detail.

As previously described, the combustion liner 24 is spaced from thehousing 22, thus providing a space 60 therebetween. Through this space60 passes a small amount of pressurized air from the plenum 38 to coolthe combustion liner 24 at its outer surface. In addition, if desired,the combustion liner 24 may be provided with a plurality of openings 62therein angled generally tangentially to the inner surface thereof toprovide a thin film of cooling air from the space 60 travelingtangentially to the inner surface of the combustion liner 24. A smallamount of cooling air for the turbine leaves the space 60 through anannular opening 64 and the remainder passes through circumferentiallyspaced openings 66 (FIG. 6) into the combustor exit 68. Upstream of thebaffle 26 pressurized air from the plenum 38 is circulated through achamber 70 in the interior housing liner 30 to cool the liner 30 and theturbine. Some of the air enters the interior of the housing through aplurality of circumferentially spaced openings 72 in the liner 30 toprovide a film of cooling air along the inner surface thereof. The mainair entrance to the combustion chamber 28, however, is through theannular orifice 42. This orifice is sized and aerodynamically shaped toprovide a main airstream having a desired velocity and a minimumpressure drop from the plenum 38 into the combustion chamber 28. Theburned gases leave the combustion chamber through the exit opening 68and are transferred to the turbine stage.

The following detailed description of the invention will make the natureof the apparatus and the combustion process occurring therein moreclear, reference being had particularly to FIG. 3.

The compressed air leaving the diffuser 13 has a radial velocitycomponent R_(o) and a tangential velocity component T_(o) (FIG. 4). Asthis air passes through the casing elbow and flows into the plenumchamber 38, the radial component R_(o) is turned axially and issubstantially slowed. The axial velocity component is now A₁ (FIG. 3).In spite of a small friction and turning loss, the slowing of thepressurized airstream results in a pressure rise in the plenum becauseof the short and smooth diffusing flow path. At the same time, thetangential velocity component is slowed as a result of a small frictionloss and the vortex motion of the air. This velocity component is nowT₁.

The pressurized airstream exits the plenum 38 through the plurality ofopenings 40 in the baffle 26 to provide air flow into the premixingchamber 34. Preferably, one or more rows of circumferentially spacedopenings 40 are provided angled generally tangentially to the innersurface of the baffle in a direction normal to the longitudinal axis ofthe chamber (FIGS. 5 and 7). As a result of the tangential velocitycomponent T₁ and the axial component A₁, the entering air forms a highvelocity vortex flowing axially of the chamber. The fuel enters thepremixing chamber through the bores 58 upstream of the openings 40.

As the fuel enters the premixing chamber 34, it is quickly vaporizedsince the chamber is in a region of high intensity radiation heatingfrom the combustion chamber 28. Moreover, the entering fuel is hit bythe inrushing high velocity air and is immediately atomized andthoroughly mixed in the violently swirling air. The combinedvaporization, atomization, and entrainment of the fuel produces asubstantially' homogeneous vapor phase air-fuel premixture in thechamber 34. Centrifugal forces maintain the vortical flow of thepremixture close to the inner baffle surface.

As shown in FIGS. 3, 5 and 7, the air flow direction into the plenumcoincides with the entrance direction into the annular orifice 42 andthe openings 40 in the baffle 26. As a result, the velocities are fullyutilized for acceleration of the main airstream. There are no frictionlosses of long pathways and no turning losses. The tangential velocitycomponent of the main airstream in the orifice 42 is now T₂ and theaxial velocity component has been accelerated to A₂. The air-fuelpremixture flows as a vortex axially of the engine axis 16 and isdischarged into the main airstream. As the vapor stream leaves thepremixing chamber, it is thrown tangentially and radially and isentrained axially circumferentially into the main airstream. The vectordiagram T₄ -R₄ (FIG. 6) shows the tangential and radial velocitycomponents of the air-fuel vapor stream as it enters the main airstream.The axial and radial velocity components of the air-fuel vapor streamare very small, while the tangential component is large.

As the vector diagram A₃ -T₃ shows, the axial velocity A₂ of the mainairstream is slowed to A₃ when hit by the very low axial velocityair-fuel vapor stream. However, the low tangential velocity T₂ of themain airstream is increased to T₃ when the main airstream is hit by thehigh tangential velocity T₄ air-fuel vapor stream.

This energy exchange and velocity change results in a very highturbulence and mixing of the premixed air-fuel vapor stream into themain airstream at this point to generate a combustible air-fuel mixturefor burning in the combustion chamber. About this point, because of thepresence of an excess of oxygen and entrained high temperature burnedgases, ignition occurs and combustion of the mixture begins (point P,FIG. 3).

Since the main airstream has both a tangential and axial flow component,the resulting vector flow V₃ produces a toroidal-helical flow path ofthe burning gases in the combustion chamber as the stream movestangentially across the surface of the toroidal-shaped combustion liner24. Moreover, there is a vortical recirculation zone of gases in thecombustion chamber. It is desirable that all the fuel in the mainair-fuel stream be completely burned when the stream reaches the innerend 74 of the liner 24 at which point the liner cooling air enters theexiting gas stream from the combustion chamber through openings 66. Inthe embodiment shown in FIG. 3, the true burning length, or flamelength, of the air-fuel stream equals the true helical-toroidal lengthfrom the ignition point P to the end 74. That is, since the gas flowpath follows a toroidal-helical path, the computed true flame length isabout 2.67 times h, where h is the dome height, i.e., cross-sectionaldiameter of the combustor liner 24 (FIG. 6). If the tangential componentT₃ would be eliminated or turned in a negative direction, for example,by having turning vanes in front of the annular orifice 42 so that thegas flow follows a simple toroidal path rather than a toroidal-helicalpath, the main air-fuel stream would start out substantially axially andthe burning or flame length would equal the length of an arc from theignition point P along the inner contour of the liner 24 to end 74. Thecomputed true length would then be about 1.8 times h, which is 48%shorter than in the case with a tangential velocity component T₃ in themain airstream.

In the combustion chamber are both burning and burned gases. The mainair-fuel stream or burning gases flow along a combustion flow path aspreviously described while the burned gases, being lighter, are drivento the center of the combustion chamber. In FIG. 3, the curved dottedline 76 represents the theoretical inner border line of the mainair-fuel stream. This line starts at point P and terminates normallyagainst the inner housing liner 30 at which is called a "stagnationpoint" 0. The dotted line 76 is also the theoretical outer border lineof the so-called recirculation zone of burned gases which existsinteriorly of the dotted line. Across the border line, a very intensivemass and energy exchange and mixing process continuously occurs. Thatis, mass is continuously being broken away from the slower movingrecirculation zone and dragged into the faster moving main air-fuelstream and is being continuously replaced in the recirculation zone bymass moving from the faster moving main airstream into the slower movingrecirculation zone. This phenomenon is known as "entrainment" and itbegins at the beginning of the main air-fuel stream, point P, andpenetrates with the downstream motion of the main air-fuel stream deeperand deeper therein until it has completely penetrated and reached theinner contour of the combustion liner 24. In FIG. 3, the solid line 78shows the outermost limit of entrainment, i.e., how deep the entrainmentpenetrates at any point into the main air-fuel stream.

At the same time, the entrainment penetrates with the downstream motionof the main air-fuel stream deeper and deeper into the recirculationzone. Line 80 shows how deep this entrainment penetrates at any pointinto the recirculation zone. The dotted line 82 represents a theoreticalborder between the air-fuel premixture exiting the premixing chamber 34and the recirculation zone. This border originates at a breakaway point00 towards the open end of the premixing chamber and ends at thebeginning of the main air-fuel stream, point P. The diverging limitlines 84 and 86 show how deep the entrainment penetrates at any pointinto the air-fuel premixture flow and recirculation zone.

In the diverging rolled-up space between the annular limit lines 78 and80, and the inner contour of the combustion liner 24, the combustion offuel in the main airstream takes place with a maximum limiting spaceheat release rate and with maximum flame and blowout stability. Thestirring effect of the entrainment process and the roll-up of the flamearound and partially into the recirculation zone combine to producemaximum heat concentration and minimum heat losses, which assure amaximum reaction rate. The heat losses are a minimum because the axialcombustor length and the liner surfaces are a minimum for a given flamelength and air-fuel mass flow rate. That is, the combustor length andsurface arc about one-half of the lengths and surfaces of classicalreverse flow and straight-through flow combustors. Furthermore, thepressure drop and velocity loss from the plenum into the combustorchamber and the premixing chamber are a minimum because of low wallfriction losses.

Due to the mass and energy exchange of the entrainment process and, inaddition, to some wall friction and uncontrolled parasitic turbulence,the main airstream is substantially slowed through the combustionchamber, whereas the recirculation zone is accelerated and kept in fastaxial and tangential vortical motion. This recirculation zone is filledwith completely burned gases of high temperature. Because these gasesare the lightest, they are continuously pushed toward the center of thecombustion chamber into the recirculation zone. As shown in FIG. 3, theaxial velocity component A₃ at the beginning of the main air-fuel streamis approximately twice that of the axial velocity component A₅ at theliner end 74. Dotted line 88 delineates on the y--y axis the approximateaxial vortex velocity components of the recirculation zone and mainair-fuel stream. As may be seen, the axial velocity component is zero atthe aerodynamic center of the combustion chamber torus, where line 88intersects the y--y axis. The flow lines and arrows in FIG. 3 illustratethe recirculating vortex flow interiorly of line 76 and the flow of thegas stream exteriorly of line 76 leaving the combustor through the exit68 to the turbine stage.

The combustor comprising this invention is characterized by efficientburning with minimum formation of unburned hydrocarbons and oxides ofnitrogen. This highly efficient burning action with a minimum ofunburned hydrocarbons is achieved because the substantially homogeneousvapor phase air-fuel mixture is burned in a combustion chamber having amaximum possible wall temperature and minimum surface area per pound offuel-air mass flow rate. Specifically, the combustion liner 24 of myinvention is of small surface area, double-curved and of one piece. Itis stiff and durable and can be made of advanced high temperatureresistant materials, thereby requiring a minimum of cooling whileremaining operational at elevated temperatures. The cooling required canbe simple convection cooling from the outside of the liner and, ifdesired, internal film cooling through openings 62 in the liner walltangentially to the inner contour of the liner. Accordingly, burningdoes not take place on these surfaces, thereby reducing the heat loadthereon.

The combustor heretofore described is also effective in a minimizingnitric oxide emissions because combustion occurs primarily with a leanair-fuel mixture having an equivalence ratio, defined as the actualfuel-to-air ratio divided by the stoichiometric fuel-to-air ratio, belowabout 0.6 and at a combustion temperature mostly below about 2850° F.The stirring effect of the controlled turbulence and the maximum heatconcentration resulting from the rolled-up flame assure the necessaryhigh reaction rate for flame and blow-out stability. The large amount ofexcess oxygen also prevents carbon monoxide emissions.

A further advantage of my invention is its simplicity of constructionand ease of maintenance. Referring to FIG. 2, the combustor may beopened by removing a series of circumferentially spaced bolts 90,thereby allowing removal of the downstream housing portion. Next, bywithdrawing the four radial pins 44 and the ignition plug 46, thecombustor liner 24 may bc removed. Finally, by removing the bolts 52,the interior housing liner 30 may be removed. Accordingly, easy accessis available to those parts exposed to the highest heat load wherebythey may be easily removed and repaired or replaced as needed.

Referring now to FIGS. 8, 9, 10 and 11, there is shown anotherembodiment of my invention. More particularly, there is shown aninterior housing liner 96 wherein the baffle portion 26 comprises astationary member 92 and a sleeve member 94 movable circumferentiallyand axially relative thereto. Further, the interior housing liner 96includes a second baffle 98 extending downstream and disposed radiallyinwardly of the baffle portion 26 defining therewith the annularpremixing chamber 34 and the downstream open end 99 of the premixingchamber 34 and into the combustion chamber 28.

The sleeve member 94 is U-shaped with an upstream extension 100 and adownstream extension 102. The sleeve member 94 is circumferentially andaxially movable on the stationary member 92 and is axially positionableby the female square threads 104 in the sleeve upstream end 100 whichengage male square threads 106 on the upstream end of the stationarymember 92. When at design air-fuel mass flow rate through the combustor,the face 108 of the sleeve 94 touches the face 110 of the stationarymember 92 and the radially outermost edge of the baffle downstreamextension 102 is spaced from the combustion liner 24 so that the annularmain air entrance orifice 42 is defined. Simultaneously, the openings 40into the premixing chamber 34 are in full, open position, i.e., theopenings 40 in the sleeve member 94 and those 40' in the stationarymember 92 coincide (FIG. 9).

Integrally with the extension 100 of the sleeve member 94 is a spoke andspur gear segment 112 which engages a circular tooth rack 114 embeddedin a suitable annular bore 116 in the housing 22. Accordingly, withreference to FIGS. 9 and 10, if the tooth rack 114 is pulled to theleft, the sleeve member 94 moves circumferentially to the left and theopenings into the premixing chamber 34 become partially closed by thecircumferential offset of openings 40 with respect to 40', as shown inFIG. 10. Further movement toward the left will further and completelyclose or offset the openings 40 with respect to 40'. Simultaneously withthis circumferential motion to the left, the sleeve member 94 movesaxially to the right, or downstream, because the axial position of thesleeve member 94 is controlled by the axial downstream lead of theengaged female and male threads 104 and 106, respectively. Because ofthis axial motion to the right of the sleeve member 94, the main airorifice 42 between the radially outermost edge of the downstream baffleextension 102 and the combustion liner 24 is enlarged. This enlargmentcorresponds to the passage area reduction into the air-fuel premixingchamber and assures constant air flow and pressure in the plenumindependent of varying air flow into the air-fuel premixing chamber.This is necessary for stable operation of a highly loaded aircompressor.

Should for any reason it be required to keep the air entrance passageinto the combustor constant or have it reduced, this can be accomplishedby replacing the thread system with a downstream lead by a simplecircumferentially interrupted tongue and groove system or, respectively,by a thread system with an upstream lead. Further, if the pressure inthe plenum chamber must remain constant, this is accomplished, as shownin FIG. 11, by bleeding the excess air from the plenum 38 as cooling airdirectly through a passage 118 located upstream of the plenum 38 and thepremixing chamber 34 into the combustor exit 68. When the sleeve member94 is circumferentially in the full open position for the air passagesinto the air-fuel premixing chamber, the upstream extension 100 of thesleeve 94 keeps the passage 118 closed, or almost closed, and no air, oronly a desired amount, can escape from the plenum 38 into the combustorexit 68. When the sleeve is circumferentially partly or completely inthe closed position, the upstream extension of the sleeve keeps thepassage corresponding partly or completely open and the excess air canescape from the plenum directly into the combustor exit.

If for any reason it is desired to control the bypass air flowindependent of the main air flow and the air flow into the air-fuelpremixing chamber, this can be accomplished by separating the upstreamextension 100 of the sleeve member 94 and controlling them by separategear and rack and segment systems.

As shown in FIGS. 8 and 9, the air entrance openings 40 and 40' into theair-fuel premixing chamber 34 are axially and tangentially arranged andevenly spaced from each other so that the individual air jets cannotcollide, but rather impinge the baffle 98 of the chamber 34 in a patternevenly spread over the entire hot surface thereof. This allows theindividual jet to develop its own air-fuel entrainment and recirculationzone. The arrows in FIG. 9 indicate this recirculation. When the airjets hit the hot baffle surface, they are drenched with fuel and theheat transfer to the air-fuel mixture and, consequently, the cooling ofthe baffle 98 is a maximum. As a result of this controlled heat transferand stirred turbulence, a substantially homogeneous vapor phase air-fuelstream leaves the premixing chamber through its annular exit opening 99into the combustion chamber 28.

In the combustor shown in FIG. 8, combustion occurs in two stages. Inthe first stage, combustion occurs on the fuel-rich side with no nitricoxide emissions and, in the second stage, on the fuel-lean side, againwith no nitric oxide emissions. As a result of the variable air passagesinto the air-fuel premixing chamber and the simultaneously varyingannular orifice into the combustion chamber, a constant, or any desired,air-fuel ratio in both stages of combustion can be achieved for anydesired fuel flow rate.

The first stage of combustion occurs in the air-fuel premixture streamdischarged from the premixing chamber 34 in the diverging space betweenthe annular outer entrainment limit line 120 and the annular innerentrainment limit line 122 and the downstream face of the baffleextension 102. The dotted line 124 represents, as in FIG. 3, thetheoretical border line between the air-fuel premixture stream and therecirculation zone.

For a given design fuel flow rate, the air inlet passages into theair-fuel premixing chamber can be sized so that the air-fuel premixtureequivalence ratio is about 1.35, which is fuel-rich, but well within thestable burning limit of a vapor phase air-fuel mixture. The maximumcombustion temperature at this air-fuel ratio is about 3400° F.;however, there is little or no nitric oxide emission because theavailable oxygen reacts more readily with the hydrogen and carbon thanwith the nitrogen. Furthermore, the combustion reaction rate is amaximum due to the entrainment of high temperature burned gases from therecirculation zone and the radiation heat of the closely spacedrolled-up flame of the second stage of combustion.

The air-fuel premixture stream leaves the opening 99 with a largetangential velocity component and thus travels the desired, relativelylong gas flow path length before entering the main airstream. By thetime the burning particles approach the main airstream, the combustionin the first stage is completed (line 126, FIG. 8). The computed trueflow path length of a burning particle from the ignition point P' to theline 126 is equal to about 1.5 times h, where h is again the dome heightof the combustor.

Any unburned fuel from the first combustion stage now mixes with themain airstream entering through the annular orifice 42 and the secondstage of combustion begins about at point P. The fuel-air equivalenceratio is now about 0.6 or lower with a combustion temperature of about2800° F. Despite the leanness of the fuel-air mixture, the combustionreaction rate is high due to the entrainment of the high temperatureburned gases, the radiation heat of the closely spaced high temperatureflame of the first combustion stage and minimum heat losses from thecombustor liner.

As shown in FIG. 8, the first stage combustion flame with a temperatureof about 3400° F. burns very close to the downstream face of the sleevemember extension 102, but not in contact therewith. Rather, an air filmwhich is formed by air flowing through, and spilling out of, a series ofspaced grooves 128, which are continuously replenished by air from theplenum 38 through passages 130 communicating therewith, flows betweenthe face and the flame, thereby drastically decreasing heat flow to theextension 102. The flame side of this air will burn, but not itsboundary layer flowing along the face. The heat entering the extensionis rejected by convection partly from the upstream face of the baffleextension into the fast-moving cooler air in the plenum, and partly fromthe inner surfaces of the grooves 128 into the fast-moving cooler air inthe grooves. Since the air in the plenum chamber maintains a hightangential velocity T₁, as described earlier, and continuously strikesthe entire upstream face of the extension with this velocity, the heattransfer from the baffle extension to the cooler air is excellent, as isthe cooling of the baffle. The air from the plenum chamber approachesthe tangential entrance passages 130 into the groove 128 with the fullvelocity T₁ and in the passages 130 gets accelerated to a hightangential velocity as a result of the pressure drop from the plenuminto the combustion chamber. The area of the tangential entrancepassages 130 into the grooves 128 is a small fraction of the axialpassage area of the grooves 128. Therefore, the axial velocity in thegrooves 128 is very small and the air motion is completely controlled bythe centrifugal field of the tangential velocity component. Therefore,the air moves on the inside of the grooves with a high tangentialvelocity along the radially outer surface of the grooves until itreaches their downstream end where it turns smoothly into the faces andcrosses them tangentially being wedged between the face and the flame.By properly choosing the number of grooves and the axial and radialdepth for a given cooling air mass flow rate and flame temperature, themetal temperature of the baffle can be closely controlled over a widerange of operating conditions. Further, since a static load on thebaffle is small and since it may be made of advanced high temperatureresisting materials, the allowable metal temperature of the baffle canbe high and the required cooling air mass flow rate low.

However, should it be desired to reduce the heat load on the baffle,this may be accomplished as shown in FIG. 12 by forming the first stagecombustion a suitable distance downstream of the baffle. In thisembodiment, it is preferable that the exit opening 131 from the air-fuelpremixing chamber 34 be in the form of individual circumferentiallyspaced openings rather than a continuous annular orifice as heretoforedescribed. This results in individual air-fuel streams which generateindividual entrainment patterns about themselves. They are spaced farenough apart to leave between them passageways so that burned hightemperature gases from the downstream recirculation zone in thecombustion chamber can easily pass between them to the upstream sidethereof and fulfill the entrainment requirement about them. As a result,the reaction rates in these individual air-fuel streams are a maximumand the combustion time of the available oxygen and the flame lengthsare a minimum.

However, it is possible to maintain an annular flame spaced downstreamfrom the baffle. This flame will be stable if the cooling air flowthrough the baffle and into the upstream side of the flame is increased.The entrainment of the cooling air flow along the baffle will produce anupstream recirculation zone, as shown by the arrows in FIG. 12.

FIG. 12 illustrates a baffle that allows for the control of cooling airflow simultaneously with the air flow into the premixing chamber 34. Inthe baffle, a radially innermost cooling air groove 132 is locatedimmediately beneath and in the face of the baffle member 92 and isconnected to the plenum 38 by tangentially arranged passages 134 in thesleeve member 94 and coinciding passages 134' in the baffle member 92.In addition, the passages 134, 134' are arranged relative to the passage40, 40' from the plenum chamber 38 into the air-fuel premixing chamber34 such that when the passages 40, 40' are in a desired open position,e.g., fully open, the passages 134, 134' are in a correspondingly fullyopen position. This allows for the control of cooling air into theentrainment space between the baffle and the flame as a function of fuelflow.

FIG. 13 shows a longitudinal section of another embodiment of myinvention that allows for the control of the main airstream from theplenum into the combustion chamber, the flow of the cooling air aroundthe combustion liner, and the flow. of excess air from the plenum intothe combustor exit all simultaneously with the control of air flow intothe air-fuel premixing chamber. FIG. 13 is similar to the combustorshown in FIGS. 8 and 12; however, in the combustor shown in FIG. 13, anadditional liner 140 has been interposed between the housing 22 and thecombustor liner 24, thus defining a cooling air space 142 between theliner 24 and the liner 140 and a corresponding bypass air space 144between the liner 140 and the housing 22. The air spaces 142 and 144communicate with the plenum 38 through a plurality of circumferentiallyspaced openings 146 and 148, respectively. Likewise, the main airstreamfrom the plenum 38 to the combustion chamber 28 enters through aplurality of circumferentially spaced orifices 150. In addition, thedownstream extension 102 of the baffle sleeve member 94 carries aplurality of radially-extending fingers 152 operative to close off theopenings 146 and 148 and the orifices 150.

The main air entrance orifices 150, the openings 146 and 148 to the airspaces 142 and 144, respectively, and the fingers 152 are sized andspaced such that at the full air flow through the opening 40, 40' intothe air-fuel premixing chamber 34 there is full air flow to thecombustion chamber 28, the cooling air space 142 and to the entrainmentspace between the first stage flame and the downstream face of thebaffle extension 102 through the openings 128 and 134, 134', with littleor no air flow through the bypass space 144 (FIG. 13). If the fuel flowinto the premixing chamber 34 is reduced, then the sleeve 94 is rotatedby the gear rack 114 and the mating gear segment 112, as earlierdescribed, partly closing off the openings 40, 40' into the premixingchamber 34 and the openings 134, 134' from the plenum into theentrainment space adjacent the first stage flame to provide the desiredair-fuel ratio for the first stage of combustion. Simultaneously, theorifices 150 are partially closed off by the fingers 152 to reduce themain air flow to the combustion chamber to provide the desired air-fuelratio for the second stage of combustion with the reduced fuel flow. Inaddition, the openings 146 are either partially or completely closed offto reduce the flow of cooling air behind the liner 24 and thereby reduceheat losses from the lean second stage of combustion. This preventsearly blowout of the second stage combustion, especially at altitudeswhere the ambient pressure and temperature are low. Still further, theopenings 148 to space 144 are open to bleed the excess plenum air behindliner 140 to the combustor exit 68 to maintain the desired pressure inthe plenum 38.

The embodiments of my invention shown in FIGS. 15 and 16 provide for arelatively long first stage flame as may be desired, for example, forslow burning fuels. Referring first to FIG. 16, it may be seen that thecombustion liner 24 is so shaped and placed with respect to the housing22 that the air-fuel premixture exiting the premixing chamber 34achieves the desired long flow path length before colliding and mixingwith the main airstream entering the combustion chamber through opening160. The dotted line 162 represents the theoretical borderline betweenthe premixture stream and the main airstream while the lines 164 and 166are the theoretical entrainment and mixing limit lines.

In spite of the lean air-fuel ratio in the second stage of combustion,the combustion reaction rate is high due to the presence of hightemperature burned gases and the radiation heat of the closely spacedfirst stage of combustion. Further, the downstream end of the liner 24and the housing 22 produce a recirculation zone. A dotted line 168 whichstarts at the annular breakaway edge 170 and ends at the stagnationpoint 0 is the theoretical borderline between the main air-fuel streamand the recirculation zone. The lines 172 and 174 are the theoreticalentrainment and mixing limit lines. Since the recirculation zone isfilled with burned high temperature gases, a second annular ignition andflame anchoring zone of the second stage of combustion is generatedbeginning at the breakaway edge 170. The same results can be achieved inthe embodiments of my invention described in relation to FIGS. 3, 8 and12 by shaping the combustor liner 24, such that an annular, downstreamextending space is provided for a second recirculation zone before thegas stream enters the exit 68.

The embodiment shown in FIG. 15 differs from that shown in FIG. 16 inthat the combustion liner 24 is provided with a plurality of generallyaxially extending passages 178 of suitable size, number and arrangement,as shown in FIG. 17, to allow the main airstream to enter the combustionchamber in the form of individual jets with which the air-fuelpremixture collides and mixes. The outer and inner limit lines ofentrainment and mixing of the air jets are shown at 180 and 182,respectively. Because of the concentric arrangement of the passages 178and the attendant entrainment pattern, a recirculation zone ofcompletely burned high temperature gases is produced which together withthe entrained high temperature gases and the radiation heat from theclosely spaced first combustion stage ignites and anchors the leansecond stage combustion flame. about at the breakaway edges 184 of themain air entrance passages 178 in the liner 24.

Accordingly, it may be seen that the embodiments shown in FIGS. 15 and16 in comparison with that shown in FIG. 13 moves the breakaway edge,point P, downstream to delay the collision and mixing the air-fuelpremixture with the main air-stream for a desired time or for a desiredgas flow path length. Then film cooling of the inner contour of thecombustion liner is provided through the openings 186. Further, air flowcontrol is accomplished by the movement of the radial fingers 152 in themanner described in relation to FIG. 13. The bypass air throughpassageway 118 is controlled in the manner described in relation to FIG.11.

FIG. 18 shows schematically an application of my invention to an annularstraight flow combustion chamber integrated between an axial compressor200 and an axial turbine 202 about the longitudinal axis 204 of a largegas turbine engine. It may be seen that the combustor comprises ahousing 206, a combustion liner 208 mounted within the housing 206 inspaced relation thereto defining a combustion chamber 210, and aninterior radial strut system 212 supporting three concentricallydisposed annular air-fuel premixing chambers 214, each having a closedupstream end 216 and an opening downstream into the combustion chamber210. Surrounding the premixing chambers 214 is a plenum 218 forreceiving pressurized air from the compressor 200. The plenumcommunicates with the air-fuel premixing chambers 214 through aplurality of openings 220 in the radially inner and outer baffleportions 222 and 224, respectively, of each chamber 214. The plenum 218further communicates with the combustion chamber 210 through threeconcentric annular orifices 226 defined by the outer circumferentialsurfaces of the downstream ends of the outer baffle portions 224 and theinner circumferential surfaces of either the adjacent combustor liner208 or inner baffles 222. Fuel is injected into the premixing chambers214 through the fuel entrance bores 228 at the closed upstream ends 216of the air-fuel premixing chambers 214.

Pressurized air from the compressor 200 enters the plenum 218 and passesinto the air-fuel premixing chamber 214 through the openings 220 in thebaffle portions 222 and 224, the openings 220 being angled generallytangentially to the inner surfaces thereof. The entering pressurizedairstreams create high velocity vortices flowing axially of thepremixing chambers 214 which atomize and entrain the fuel injected atthe upstream ends 216 thereof to produce substantially homogeneous vaporphase air-fuel premixtures. The premixtures then exit the premixingchambers through the concentric annular orifices 230 and begin burning.The mixtures are fuel-rich but well in the stable burning limit becauseof the presence of the high temperature entrained gases. Pressurized airfrom the plenum 218 also enters the combustion chamber through theconcentric annular orifices 226 to provide the corresponding mainairstream for each air-fuel premixture stream. As heretofore disclosedand described, the unburned air and fuel from the air-fuel premixturestream enters the main airstream with a high tangential velocitycomponent and is circumferentially and axially entrained therein. Due tothe radially close spacing of the burning air-fuel streams and therecirculation zones with their respective stagnation points O, theair-fuel streams burn as fast and stable and contamination-free as inthe embodiments previously disclosed. The radial spacing between theair-fuel premixture chambers and main air orifices into the combustorare chosen such that the true burning lengths of the three air-fuelpremixture streams before entering the main airstreams are about equal.This choice of radial spacing also allows for a given pressure drop intothe combustor and equal axial stream velocities and mass flow rates forthe three streams.

The arrangement of the air-fuel premixture chambers in the combustorshown in FIG. 18 has the advantage that the air entrance openings 220can be arranged in the tops and bottoms of the premixture chambers.Furthermore, in such a multiple premixture chamber arrangement, there isno need for mechanical devices to reduce the air mass flow to thepremixture chamber and the main airstream flow to maintain a constantair-fuel ratio with reduced fuel flow and engine output. Rather, this isachieved by simply cutting out the fuel flow to one or more of thepremixture chambers. For example, fuel flow may be cut first to theradially inner chamber to reduce engine output and then to the radiallyouter chambers for further reduction or to idle the engine. This can bedone because the air-fuel streams of the various chambers and mainairstreams are separated from each other by the recirculation zones.

Thus having described the invention, what is claimed is:
 1. A combustorfor generating a hot gas stream by the combustion of fuel in apressurized air stream, said combustor comprising:a housing; acombustion zone therein; a plurality of radially spaced air-fuelpremixing chambers each having an upstream end and opening downstreaminto said combustion zone; means for injecting fuel into said premixingchambers at said upstream end independently of each other; and acompressed air inlet plenum surrounding said premixing chambers andseparated therefrom by baffles, said plenum communicating with saidpremixing chambers through openings in said baffles to generate asubstantially homogeneous vapor-phase air-fuel premixture in each ofsaid premixture chambers; said plenum further communicating with saidcombustion zone through a plurality of radially spaced orifices adjacentthe outer periphery of said baffles providing for the entry of mainstream outwardly circumferential of each of said premixing chambers atthe downstream end thereof; said premixture entering said main airstream and having a substantial tangential gas flow velocity componentwhen introduced therein such that said air-fuel premixture issubstantially instantaneously intermixed into said main air stream togenerate a combustible air-fuel mixture for burning in said combustionzone; the main air streams being radially spaced and having a vectorcomponent of flow effective to generate overlapping vorticalrecirculation zones of burned gases.
 2. A combustor for generating a hotgas stream by the combustion of a pressurized premixed air-fuel stream,said combustor comprising:a housing; a combustion zone therein; aplurality of radially spaced air-fuel premixing chambers upstreamthereof, each chamber being formed by a baffle having an upstream endand a downstream orifice into said combustion zone; means for injectingfuel into said premixing chambers at said upstream ends thereofindependently of each other; and a compressed air-inlet plenumsurrounding said premixing chamber and separated therefrom by saidbaffles; means interconnecting said plenum with said premixing chambers,whereby pressurized air flows from said plenum into said premixingchambers to form a substantially homogeneous vapor-phase air-fuelmixture in each of said premixing chambers; said downstream orifices ofsaid air-fuel premixing chambers being radially spaced and dischargingradially spaced premixed air-fuel streams into said combustion zone,said streams due to their spacing having a vector component of floweffective to generate a vortical recirculation zone of burned gases inthe space between said premixed air-fuel streams within said combustionzone.
 3. A combustor for generating a hot gas stream by the combustionof a pressurized premixed air-fuel stream, said combustor comprising:ahousing; a combustion zone therein; a plurality of radially spacedair-fuel premixing chambers upstream thereof, each chamber being formedby a baffle having an upstream end and a downstream orifice into saidcombustion zone; means for injecting fuel into said premixing chambersat said upstream ends thereof independently of each other; and acompressed air inlet plenum surrounding said premixing chambers andseparated thereof by said baffles; said plenum communicating with saidpremixing chambers through openings in said baffles providing for theentry of air to generate a substantially homogeneous vapor-phaseair-fuel premixture in said premixing chambers; said downstream orificesof said air-fuel premixing chambers being radially spaced anddischarging radially spaced premixed air-fuel streams tangentially intosaid combustion zone, said streams due to their spacing having a vectorcomponent of flow effective to generate a vortical recirculation zone ofburned gases in the space between said premixed air-fuel streams withinsaid combustion zone.
 4. A combustor of claim 2 wherein:an arcuatecombustor liner is mounted within said housing in spaced relationthereto and defining said combustion zone, said air plenum communicatingwith the space between said liner and said housing, whereby combustorliner cooling air flows from said plenum downstream along the outside ofsaid liner, the premixed air-fuel stream neighboring said liner beingspaced thereof and having, due to its spacing, a vector component offlow effective to generate a vortical recirculation zone of burned gasesin the space between said premixed air-fuel stream and said liner; saidrecirculating burned gases flowing upstream along the inside of saidliner in counterflow to said cooling air flow.